Huntington's disease (HD) is an inherited neurodegenerative disorder that affects the lives of more than 100,000 people in the US. The complexity of the chronic symptoms and pathology of Huntington's disease (HD) has long puzzled researchers and prevented the progress of therapeutic intervention. However, the primary cause of HD is genetically simple; expanded HD CAG repeats that encode an expanded polyQ region in the huntingtin protein. Thus, understanding the structure and function of the huntingtin protein as it relates to the disease will likely elucidate a fundamental source o HD pathology and be crucial to developing therapies. Since we have developed a series of full-length recombinant human huntingtin proteins, as a resource for structure-function studies, this grant really aims to identify the effects of polyQ expansion on full-length huntingtin in HD and to generate novel targets and discover therapeutic molecules that directly bind to the huntingtin protein and modify its functional activities. Aim 1 will define the impacts of polyQ expansion on the structure and function of the full-length huntingtin protein by using various biochemical assays and high resolution structural studies (electron microscopy, atomic force microscopy and crystallography). Aim 2 will systematically identify altered phosphorylation modifications of mutant huntingtin using the purified full-length huntingtins with different polyQ lengths because phosphorylation of mutant huntingtin has been implicated in HD pathogenesis. We will generate and validate the phospho-antibody reagents and use them to identify the specific isoforms of the phosphorylated full-length huntingtins strongly related to HD pathogenesis. Aim 3 will identify aptamers that modify the impact of the polyQ region on huntingtin structure and function. Aptamers will be used as versatile reagents for high-throughput drug screening because they bind to target proteins with a high specificity and s strong affinity and introduce structural and functional changes of target proteins as similar as by therapeutic molecules. Our results will provide a thorough understanding of the structural and functional properties of full- length huntingtin as a primal disease-cause and lead to identify therapeutic molecules which will be validated in two mice models (CAG knock-in & YAC128) and human neuronal cells differentiated from human iPS from HD patient fibroblast. These will also enable rational design of therapeutics aimed at interfering with the HD disease process before neuronal cells begin to succumb to its cumulative effects.